The Ras family of proteins is comprised of small GTPases that are subdivided into several sub-families known to be involved in diverse cellular actions, such as cell proliferation, differentiation and apoptosis. Moreover, Ras is a known oncogene, and the Ras protein is implicated in oncogenic cell transformation through complex signaling pathways that employ an increasing number of downstream effectors. Some of these effectors are included in Ras sub-families, such as, for example, the Rho subfamily of small GTPases.
The Rho family proteins are implicated in regulating diverse cellular processes as well. One prominent Rho activity comprises effecting actin cytoskeletal organization. Such activities include regulating cell shape, cell attachment and adhesion, cell motility and invasion, cell-cell interactions, cell proliferation, differentiation and apoptosis. For example, Rac1; RhoA and Cdc42 are all implicated in promoting cell motility and invasion. As such, these proteins may be involved in promoting motility, invasiveness and metastasis of tumor cells. In addition, disassembly of actin stress fibers is associated with malignant transformation. Moreover, another prominent and distinct Rho activity includes activation of signaling cascades that enhance gene expression through the induction of various transcription factors, resulting in cell proliferation, cell cycle progression, differentiation and apoptosis. For Example, the Rho proteins Rac1 and Cdc42 activate Jun NH2-terminal kinases (JNKs), which in turn activate Jun, ATF-2 and Elk-1 nuclear transcription factors. Rho family proteins can also activate NFxcexaB and SRF transcription factors. Virally transduced and mutated versions of cellular Fos, Jun, and NFxcexaB were originally identified as potent retroviral oncogenes implicated in various tumors and cancerous states (Bishop, J. M., Cell 64:235-238, 1991). Thus, Rho family mediated changes in gene expression likely contribute to their proliferative actions, and play a role in cell transformation and cancer. Moreover, several Rho family proteins have been shown to be important for Ras transforming activity. For reference, see Zohn, I. M. et al., Oncogene 17:1415-1438, 1998; Maruta, H. et al., Microsc. Res. Tech. 47:61-66, 1999; Aspenstrom, P. Exper. Cell. Res. 246:20-25, 1999; Banyard, J. and Zetter, B. R., Cancer and Metast. Rev. 17:449-458, 1999; and, Michiels, F. and Collard, J. G., Biochem Soc. Symp. 65:125-146, 1999.
In addition, several effector proteins are known to bind Rho family members, such as Cdc42 and Rac. The effectors that bind Cdc42/Rac have a consensus binding motif designated the Cdc42/Rac Interactive Binding (CRIB) motif (Burbelo, P. D. et al., J. Biol. Chem. 270:29071-29074, 1995). These effectors show GTP-dependent interaction with Cdc42 and/or Rac1, and may or may not show kinase activity. For Example, the Cdc42 effector, MSE55 is a non-kinase effector that specifically binds Cdc42 in a GTP-dependent manner, is localized to membrane ruffles, and induces long actin-based protrusions or cellular extensions in fibroblast cells (Burbelo, P. D. et al., Proc. Natl. Acad. Sci. USA 96:9083-9088, 1999). For other references on Cdc2/Rac and their effects on membrane ruffling, actin stress fibers, lamellipodia and the like, see, for example, Ridley, A. J. et al., Cell 70:401-410, 1992; and Nobes, C. D., and Hall, A. Cell 81:53-62, 1995. Other CRIB proteins are implicated in human disease, such as the Wiskott-Aldrich Syndrome, which is an X-linked recessive disorder characterized by thrombocytopenia, recurrent infections due to defective T- and B-cell function, and eczema. The CRIB protein Wiskott-Aldrich Syndrome Protein (WASP) is mutated in this disease, and it is also a Cdc42 effector (Symons, M. et al., Cell 84:723-734, 1996). Because these CRIB proteins influence members of the Rho family of proteins, these effectors may also play a role in cell proliferation, transformation, motility and metastasis. For reference, see Zohn, I. M. et al., supra.
Considering the importance of this family of proteins, there is a continuing need to discover new Ras and Rho family members and their effector proteins that modulate the cytoskeleton, actin polymerization, cell motility and invasion, and the like, and affect proliferation, differentiation, transformation, metastasis and apoptotic pathways. The in vivo activities of both inducers and inhibitors of these pathways illustrate the enormous clinical potential of, and need for, such novel proteins, their agonists and antagonists, for example, in cancer therapy. The present invention addresses this need by providing such polypeptides for these and other uses that should be apparent to those skilled in the art from the teachings herein.